(Ta1-xNbx)2O5 as the dielectric for metal/insulator/metal capacitors with Ru electrodes were investigated. Deposition was performed by ALD using tantalum ethoxide, niobium ethoxide, and ozone as source materials. The high-κ dielectric layer was synthesized from a binary layer (Ta2O5/Nb2O5) with self-diffusion of the seed layer, Nb2O5. The seed layer becomes effective at a thickness of over 60 Å. The dielectric deposited with 60 Å Nb2O5 crystallized at 575 °C, the lowest processing temperature required to achieve good step-coverage (86%) for future generation dynamic random access memory. A capacitance density of about 23.0 fF/μm2 was observed for seed layers of 20 and 35 Å. Capacitors with a Nb2O5 seed layer of 60 Å achieved a capacitance density as high as 44.2 fF/μm2. The reason for this disparity is that the thicker insulator crystallizes at a post-annealing temperature of 575 °C. The leakage current measured at 1 V was moderately low, near 1×10-7 A/cm2.